The present invention relates to a battery power control system, and more specifically, to a low-cost configuration and control method for a hybrid battery system which achieves both high energy density and high power density for use in an electric or hybrid electric motor drive system such as used in electrically propelled vehicles.
Propulsion systems for electric motor propelled vehicles (xe2x80x9celectric vehiclexe2x80x9d or EV) generally use rechargeable traction batteries to provide electric power for driving electric motors coupled in driving relationship to wheels of the vehicle. For example, U.S. Pat. No. 5,373,195 illustrates a system in which the traction batteries are connected to a direct current (DC) link, which link connects to a power control circuit such as a pulse width modulation (PWM) circuit for controlling power to a DC motor or to a frequency controlled inverter for controlling power to an alternating current (AC) motor. Hybrid electric vehicle (HEV) propulsion systems are constructed similarly to EV propulsion systems but also include internal combustion engines to drive on-board generators to supplement battery power.
In general, traction batteries for electric vehicles and hybrid-electric vehicles represent a compromise between power density and energy density.
The present invention seeks to optimize the power system for an electrically propelled vehicle by the use of the combination of a high power density battery and a high energy density battery. For example, very high energy density battery technology exists in the form of, for example, zinc-air mechanically rechargeable batteries, which have been demonstrated to achieve energy densities of 200 W-hr/kg, compared to a lead-acid battery which typically achieves only 30-40 W-hr/kg. However, the power density of such zinc-air batteries is reported to be about 80-100 W/kg. In comparison, nickel-cadmium (Nixe2x80x94Cd) batteries have been developed that achieve power densities of 350 W/kg with energy densities of 45-50 W-hr/kg. Accordingly, a hybrid battery system using a zinc-air battery in combination with a Nixe2x80x94Cd battery would provide an optimal combination of high energy and high power.
One problem with using high energy density batteries in EV applications is that such batteries are not electrically rechargeable, i.e., a battery such as the zinc-air battery requires mechanical/electro-chemical recharging. Nevertheless, a system including both a high energy density battery and a high power density battery, which system would be both mechanically rechargeable and electrically rechargeable, where electrical recharge energy is not applied to the mechanically rechargeable segment of the battery would have substantial advantages in operating capacity. Further, such a hybrid battery system could include a method to capture regeneration energy in the hybrid battery configuration that would increase an EV""s or HEV""s range for a given amount of stored energy.
As discussed above, it is desirable to provide a low-cost configuration and control method for a hybrid battery system capable of achieving both high energy density and high power density in an electric or hybrid vehicle propulsion system. Towards this end, the present invention provides a method and apparatus to control the recharging of a hybrid battery which includes both a high energy density battery, such as a mechanically rechargeable battery, and a high power density battery.
The hybrid battery system in one form of the present invention includes components which prevent electrical recharge energy from being applied to the high energy density battery while being able to capture regenerative energy to be applied to the high power density battery so as to increase an electric vehicle""s range for a given amount of stored energy. A dynamic retarding function for absorbing electrical regenerative energy is used during significant vehicle deceleration and while holding speed on down-hill grades, to minimize mechanical brake wear and limit excessive voltage on the battery and power electronic control devices.
In an illustrative embodiment, the present invention comprises a hybrid battery system, which includes a high energy density battery coupled in circuit with a boost converter, a high power density battery, a dynamic retarder, and an AC motor drive. The hybrid battery system is controlled by a hybrid power source controller which receives signals from a vehicle system controller. The hybrid power source controller uses current and voltage sensors to provide feedback parameters for the closed-loop hybrid battery control functions. Recharging the high power density battery is accomplished by a combination of capture of regenerative energy from the motor drive and recharge from the high energy density battery.